Setup the VPN on Chrome OS

To get the user and server certificates, from the pfSense UI go to System / Cert. Manager.

Under the CAs tab, find the certificate used for the OpenVPN server and click the certificate export icon to export the certificate authority.

Under the Certificates tab find the User Certificate for the VPN user you want to setup. Click the box icon to export it as a P12 file.

These files need to be available on the Chrome device, you can add them to the users Google Drive, or use some other method as desired.

Install the certificates using the steps listed under “Install certificates” via the Google Support link above.
Essentially in Chrome, you need to go to chrome://settings/certificates and import the CA certificate under the Authorities tab and the user certificate under the Your Certificates tab. When it asks for a password to install the .p12 file just leave it empty.